 |
Slide 1 |
The word laser is an acronym for light amplification by stimulated emission of radiation. Lasers operate in the quantum world of atoms and photons. Three concepts involving the interaction between photons and atoms are important for the functioning of lasers. These three concepts are stimulated absorption, spontaneous emission, and stimulated emission.1
In the classic view of an atom, there is a nucleus with a series of electrons orbiting about it. Atoms prefer to be in the resting state, which means that the negatively charged electron cloud is in harmonious balance with its positively charged nucleus. Atoms tend to stay in the resting state, unless they absorb energy from their surroundings.
A photon carries energy proportional to its frequency. If an atom absorbs a photon, then the atom moves into an excited state. Excited atoms contain electrons that have been forced into a higher orbit or energy level. Excited atoms are unstable and are driven to return to the resting state. This process, which is known as stimulated absorption, is shown in Slide 1. Stimulated absorption also can be accomplished through electrical discharge and chemical reaction.
Atoms in an excited state are unstable and need to return to the resting state. For this to occur, excited atoms must give up energy, which is released in the form of a photon. Photons carry energy away from the atom and, generally, the photon can travel in any direction. Slide 1 illustrates spontaneous emission.
Stimulated emission differs from spontaneous emission in that it is initiated by a passing photon. The photon causes the excited atom to release its excess energy in the form of a second photon. The new photon has common features with the initiating photon. First, the released photon travels in the same direction as the original photon. Second, the released photon is coherent (see The Physics of Light) with the original photon. Coherence means that the light waves formed by these photons can experience interference. Slide 1 illustrates stimulated emission.
|
Lasers are composed of three components including an active medium, a pump, and a resonating cavity. The active medium is the material that contains the atoms that will be stimulated to lase. The medium can be gaseous, liquid, or solid material. Lasers typically are named after the material that makes up the active medium. For example, HeNe lasers are common red lasers that have a gaseous mixture of helium (He) and neon (Ne) as their active medium. The pump puts energy into the active medium. Pumps can be flash lamps, electrical discharges, or other lasers. The resonating cavity consists of two mirrored surfaces at either end of the active medium. Slide 2 shows a typical arrangement for a laser.
The pump introduces energy into the active medium, which causes atoms in the medium to go into an excited state. Because these atoms are in an excited state, they are unstable and will return to a resting state through spontaneous emission. A photon spontaneously released by an excited atom can interact with other excited atoms and cause stimulated emission. In this manner, a single spontaneously emitted photon can stimulate a second photon, and these two photons then can stimulate two additional photons, and so on. Generally, the photons can travel in any direction, but the photons generated by stimulated emission always travel in the same direction as the stimulating photon. Some of these photons will travel in the direction of the axis of the active medium and strike one of the mirrors of the resonating cavity. These photons then are returned through the active medium, where they can further stimulate additional photons. Meanwhile, the pump is either continuously or systematically exciting resting atoms to ensure a constant supply of excited atoms for these photons to stimulate. Thus, photons oscillate between the two mirrors and, with each pass, additional photons are released. To extract energy from the laser, one of the mirrors is not perfectly reflecting. This mirror therefore will transmit some of the photons striking it. The transmitted photons exit the laser and can be used for a variety of applications.
|
Materials used for lasers typically are slow to spontaneously emit. This gives the oscillating photons a chance to cause stimulated emission before an excited atom can decay through spontaneous emission. The properties of lasers, therefore, are highly dependent on the material used for the active medium. The wavelength that a laser emits is dependent on the energy levels of the atoms comprising the active medium. It is possible for a laser to emit multiple wavelengths simultaneously, but the resonating cavities typically are designed to reinforce a single wavelength and suppress other wavelengths. Thus, lasers typically are considered to be monochromatic (i.e., a single wavelength).
In addition to being monochromatic, lasers have other properties that make them very useful. First, they are extremely directional, meaning that the beam emitted from lasers will remain concentrated and can travel in a straight line over vast distances. Second, because the beam is narrow as well as directional, it can be focused to an extremely small spot. Thus, intense levels of energy can be concentrated to a small spot.
The size of a laser cavity depends on multiple factors. First, the length of the cavity in conjunction with the gain medium controls the wavelengths that can be emitted. Generally, the laser can support more wavelengths or longitudinal modes with a longer cavity. Second, a larger cross-section of the laser cavity provides more area for the active medium to absorb energy from the pump. Furthermore, larger cavities provide more atoms in the active medium and potentially more photons in the output beam. The latter, however, depends on the efficiency of the laser.
Continuous Wave and Pulsed Lasers
Some lasers operate in a continuous wave (CW) mode. These lasers emit light in a continuous stream and typically have relatively low power. Conversely, pulsed lasers can have extremely high powers, but the light is emitted from the laser in a discrete pulse lasting only fractions of a second. Pulsed lasers operate by Q-switching, in which the resonating cavity is designed to contain the photons and let the numbers build. The cavity then is opened to release all of the photons in a large burst. The cavity then can be closed again and the photon build up repeated.
Laser output is typically given in terms of watts (W) or joules (J). Joules are a unit of energy that is directly proportional to the number of photons emitted. Watts are a unit of power and are the number of joules emitted in a given second. Pulsed lasers are specified in terms of joules because a single laser pulse contains a finite amount of energy or number of photons. Continuous wave lasers are specified in terms of watts. Because they continuously emit photons, the total amount of energy emitted by the laser continuously increases. Therefore, it does not make sense to compare continuous wave lasers based on energy because there is no natural unit or finite amount of energy emitted. However, if the number of photons emitted in a fixed time interval is measured, then a comparison between continuous wave lasers is possible. High-power continuous wave lasers emit more photons in a given second than low-power lasers.
Photocoagulation, Photodisruption, and Photoablation
In ophthalmic applications, laser energy interacts with ocular tissue in one of three different ways. The first mechanism is photocoagulation. The laser light is absorbed by pigment within the treated tissue, which causes a localized heating. This heating can cause coagulation of blood and shrinkage of collagen. The heating tends to damage or burn surrounding tissue.2-3
In the second mechanism, photodisruption, laser energy that is focused to a small spot within the tissue strips electrons of the atoms in the tissue to form a plasma. The plasma rapidly expands, forming a hole in the tissue and a shock wave that propagates outward from the site of focus. The shock wave can damage surrounding tissue, so much effort has been made to reduce the effects of the damaging wave. The damage may be minimized by shortening the duration of the laser pulse. Typically, pulses lasting nanoseconds (10-9 seconds), picoseconds (10-12 seconds), and even femtoseconds (10-15 seconds) are used. Photodisruption is usually independent of the absorption of the material and is more dependent on the location the laser energy is focused.2-3
In the third mechanism, photoablation, the energy of the laser is sufficient to break molecular bonds, causing them to simply fall apart. Photoablation causes little damage to the surrounding tissue, so lasers that remove tissue through photoablation sometimes are called "cold" lasers.2-3
A variety of lasers exists for ophthalmic procedures. Some of the more common types are listed below along with the typical application and wavelength. The list is not exhaustive and additional types of lasers and applications are continuously emerging.
Excimer lasers are a class of laser, as opposed to a type of laser. Excimer is a contraction for excited dimer. The most common type of excimer laser is the argon fluoride (ArF) laser, which operates in the ultraviolet at a wavelength of 193 nm. An argon fluoride laser is used for refractive surgery because it can photoablate tissue and provide precise removal of corneal tissue with minimal damage to the surrounding tissue.4
The argon (Ar) laser has been used widely within the ophthalmic community. Two wavelengths, a blue-green 488 nm and a green 514 nm, are commonly used for retinal photocoagulation. This laser is used to treat abnormal blood vessels and neovascularization in diabetic retinopathy and macular degeneration.5
The krypton (Kr) laser is used for similar applications as the argon laser. This laser provides two additional wavelengths for retinal photocoagulation — yellow 647 nm and red 676 nm. The choice of whether to use argon or krypton is based on the absorption of the tissue being treated.6
The neodymium:yttrium aluminum garnet (Nd:YAG) laser is a solid-state laser that operates at 1,064 nm in the infrared. It typically is used to treat posterior capsule opacification following cataract surgery. The Nd:YAG laser operates by photodisruption and creates holes in the opacified tissue to restore vision in these patients.7
Holmium:YAG lasers are finding applications in ophthalmology, with the most recent being laser thermal keratoplasty (LTK). These lasers operate at a wavelength further into the infrared at 2,100 nm. Corneal tissue will absorb this wavelength. The localized heating will cause collagen shrinkage and subsequently a change in the shape of the cornea.8
Erbium:YAG lasers have been explored for use in cataract surgery. These lasers operate at a wavelength of 2,940 nm, where water has a high absorption rate. Endoscopic probes with fiber optic delivery of the erbium laser energy have been used to emulsify the crystalline lens, and this technology may provide an alternative to traditional ultrasonic phacoemulsification.9
As stated above, this list is not exhaustive. Other applications of lasers in ophthalmology include trabeculectomy, sclerectomy, iridectomy, and intrastromal ablation. Other laser types also are being explored, such as diode lasers that offer more compact and portable housings and eliminate the bulky cooling and electronics needed in some of the preceding lasers. Multiple wavelengths are being explored for retinal photocoagulation to treat different levels within the retina and choroid. Of all the areas of medicine, lasers have been most applicable in ophthalmology. This is a trend that is likely to continue.
